Contents 7 Boiler Scale

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

Silicate scales can form readily in all boilers. Maintenance of a silica content to caustic alkalinity ratio of less than 0.4 1 prevents this. 

The use of hydroxyacetic/formic acid in the chemical cleaning of utility boilers is common. It is used in boilers containing austenitic steels because its low chloride content prevents possible chloride stress corrosion cracking of the austenitic-type alloys. It has also found extensive use in the cleaning operations for once-through supercritical boilers. Hydroxyacetic/formic acid has chelation properties and a high iron pick-up capability thus it is used on high iron content systems. It is not effective on hardness scales. 

Several processes have been developed for coal liquefaction. Large-scale pilot plants have been in operation for the solvent-refining coal (SRC) process, and a pilot plant is being constructed for the H-Coal process, which is a direct catalytic process. Construction of demonstration plants is under consideration. The coal liquids produced from the current processes contain large amounts of residual fuels. They probably will be used initially as boiler fuels for stationary power plants. However, the nitrogen content of coal liquids is much higher than the petroleum residual fuels. The sulfur contents of coal liquids can vary considerably they depend on the type of coal and the liquefaction process used. Current coal liquefaction processes are capable of produc-... 

The principle advantage of dry extruder preparation is that no expensive stacked-tray cookers or steam boiler are required, and the total capital investment for facilities under 100 tons per day in size is significantly less than for traditional full-press or solvent extraction processes. Therefore, this means of preparation is commonly applied in very small-scale plants commonly referred to as mini-mills. The primary disadvantage of dry extruder preparation is that the meal produced is higher in fat content than many poultry producers desire, and lower in bypass protein than many cattle producers desire. Thus, meal produced from a full-press mill using dry extrusion preparation generally obtains a lower price than meal produced from a traditional full-press operation. 

Agglomeration and The Content of Amorphous Material in FB Combustion. A Full-Scale Boiler Test... 

Unalloyed steels can be used in air up to 550°C and low-alloy steels up to approximately 600°C. The applicability of high-alloy steels is determined by the alloy contents, with special importance attached to Cr, Si, and Al, as demonstrated in Figures 20.48,20.55, and 20.56. Water vapor and carbon dioxide in air generally worsen the scaling behavior of steels. The resistance of steels in water vapor is of particular importance in steam boilers and heat exchangers. It has been investigated in the literature at temperatures up to 800°C. 

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